In order to write high quality digital images, it is necessary to insure that the spacing between the lines of the image is kept uniform. Otherwise, image defects known as "banding," caused by small variations in the spacing between lines of an image will become visible in the resulting image. The human eye is very sensitive to this defect and noticeable banding appears at variations as small as 0.1%. Therefore, in the design and manufacture of high quality digital printers, great care is taken to assure that the spacing between lines is uniform.
One type of digital printer uses a rotating mirror such as a polygon to scan a modulated laser beam onto a line on a medium such as photographic paper. The medium is moved slowly in the "cross-scan " direction to place each successive line adjacent to the previous line. To achieve band free images, the velocity of the medium in the cross scan direction, sometimes called the slow-scan direction, must be kept constant. Variations of this velocity are known as flutter, and flutter in excess of 0.1% can cause visible banding. To insure that flutter performance in the slow scan drive apparatus will not cause banding, great care is taken in the design and manufacture of all the components of the slow scan subsystem. Precision bearings and couplings, low runout shafts and rollers, precision drive motor, encoder, and other components are used to assure acceptable flutter performance.
A typical slow scan media drive apparatus 40 is shown in FIG. 1. The medium 41 is engaged in nip 10, which is formed by loading pressure roller 11 against drive roller 13 by force 12. Motor 14 causes roller 13 to rotate, and slowly pull the medium 41 in the direction shown by arrow 42. Upstream of nip 10 is another nip 20. Nip 20 is formed by loading pressure roller 21 against tensioning roller 23 by force 22. Motor 24 applies opposing torque to keep the medium 41 under tension between nip 10 and nip 20. Writing of an image occurs at scan line 30, between nip 10 and nip 20, as the medium is pulled past platen roller 31.
The field of writing digital images on lenticular media is relatively new. One of the problems posed by writing digital images on lenticular media is that one surface of the lenticular material is "bumpy ". FIG. 2 shows a lenticular medium 43 being conveyed through a slow scan drive apparatus 40. Lines of an image are written on the smooth underside 44 of the medium 43. The top surface 45 of the lenticular medium is covered by lenticules 51. The size of the lenticules has been exaggerated for clarity. As the lenticules 51 pass through nips 10 and 20, they cause pressure rollers 11 and 21 to lift and fall a small amount. If the lenticules are large, the magnitude of the lift and fall will be large and if the lenticules are small, the magnitude of the lift and fall will be small. In either case, the passage of the lenticules through the nips will result in some amount of variation in the height of the pressure rollers. This raising and lowering of the pressure rollers produces a variation in speed of the medium as it is being conveyed through the apparatus. Even small disturbances such as those caused by the lenticules going through the nips are capable of degrading the flutter performance of the slow scan drive sufficiently to result in banding in the scanned image.
It is desirable to provide a system for conveying lenticular material through a slow scan media drive apparatus which minimizes flutter caused by lenticules.